1. Field
Example aspects of the invention relate generally to reducing distortion from electrical devices, and more particularly to an improved system and method for reducing or canceling nth-order intermodulation products of nonlinear electronic devices.
2. Description of the Related Art
Nonlinear electronic devices are ubiquitous in modern technologies and find myriad uses in telecommunications, computing, and military applications. Many applications of these devices require a linear response, i.e., to achieve maximum performance, the output signal from the device must be proportional to the input signal. One example of linear performance desired from a nonlinear device is the amplification of a telecommunications signal by a power amplifier. In this example, the signal is a band comprised of multiple frequencies or channels. Because this signal must share the radio frequency (RF) spectrum with other signals, linear performance is required from the amplifier because any nonlinear amplification creates frequencies outside the signal band, which may contaminate or interfere with other telecommunications signals, resulting in signal degradation and increased error rates.
At a low power level, the response of a typical amplifier is approximately linear; the nonlinear distortion of an amplifier at low output power is often negligible. However, as the output power increases, so does the nonlinear distortion. One source of distortion is the intermodulation, or intermixing, of the different frequency components comprising the input signal. Intermodulation generated by the nonlinearities in the amplifier results in intermodulation products, which can increase exponentially with the amplifier gain. At high output power, the output signal of a nonlinear amplifier may be substantially compromised by these intermodulation products. In turn, these intermodulation products can interfere with other signals outside the frequency band of the input signal. However, high output power is often necessary to meet performance requirements, such as cost, power consumption, and device footprint.
In some instances, intermodulation products resulting from the high-gain amplification of an input signal may be removed from the output signal by signal filtering techniques, such as the use of high-pass, low-pass, or band-pass filters. However, certain intermodulation products occur at or near the frequency of the input signal. These distortions often prove difficult to remove from the output signal. For example, some third-order intermodulation products are comprised of frequencies comparable to the input signal frequencies; bands of these third-order intermodulation products can overlap with and fall outside of the desired linear amplification of the input signal. Thus, the aforementioned signal filtering techniques cannot be easily used to filter these intermodulation products from the desired amplified output.
For applications requiring high amplifier gain, there are two common schemes for improving amplifier linearization: predistortion and postdistortion. In a predistortion approach, a nonlinear amplifier is modeled or measured in order to characterize the nonlinear response of the amplifier. A predistortion circuit is then designed which compensates the input signal for the intermodulation products, or distortion, that will be added to the signal upon amplification. Thus, the circuit predistorts the input signal; when the predistorted signal is amplified, the result is an output signal nearly linear with the original input signal. Postdistortion is analogous to predistortion but the compensation for the amplifier nonlinearities is added to the signal after amplification.
Both of these methods have limitations, including complex pre- or postdistortion circuitry, high power consumption, and limited actual reduction of the intermodulation products. Another approach to improving amplifier linearization is the use of feedforwarding techniques. However, these also require complicated circuits and increase total amplifier cost. Additionally, feedforward circuitry can be difficult to control, particularly at high frequencies. Through comparison with the example embodiments set forth below, further limitations of the above-mentioned approaches will become apparent.